This invention relates generally to the field of sensors for the measurement of level of liquid or granular solid in an array of containers where it is physically not possible to insert a probe inside each container, surround the container with launching electrodes, or attach sensors to walls. More particularly, it relates to the field of level sensors where an electromagnetic wave is launched from the outside of the container and the reflected signal processed to extract the information about liquid level.
Level of liquid or granular solid inside containers e.g. bottles, vials etc. need to be monitored for hospitality, pharmaceutical, healthcare, industrial and other areas. A common example is monitoring the amount of drink inside beverage bottles in bars. Every year a significant amount of beverage is lost due to shrinkage that needs to be checked. Furthermore, multiple beverage containers, located in trays and shelves need to be monitored in an economic and timely fashion for inventory.
Prior art e.g. weighing of individual containers to determine content is an expensive proposition since as many force sensors are required as the number of containers. Capacitive or transmission line sensors are economic but need conducting electrodes around the containers. Invasive techniques e.g. inserting a probe in the container are not acceptable due to cost and inconvenience.
Prior art U.S. Pat. No. 6,564,658 teaches the use of slow-wave structures to measure liquid level and teaches the confinement of electromagnetic energy in a small volume. One embodiment of the invention teaches liquid level measurement by placing the electrodynamic element outside the container. However, the fields generated by the electrodynamic element can attain only partial penetration and not throughout the bulk of the liquid. As a result, this method is unlikely to provide the adequate sensitivity in many applications.
Thus, a better solution is needed to accurately measure level of liquid or granular solid inside containers where neither electrodes or sensors around the container cannot be used, nor a probe be inserted. Furthermore, the solution needs to amenable to measuring multiple containers in an array and yet be economic. All of the said features are provided by the following invention.